There is convincing evidence to indicate that the preovulatory surges of LH and FSH in rat plasma are caused by a properly timed, nearly constant rate release of hypothalamic LHRH for about three hours followed by a period of diminished LHRH release. It has been proposed that during this event LHRH primes the pituitary to itself to release markedly increased quantities of LH and FSH in response to further stimulation with LHRH. Both in vivo and in vitro experiments are designed to show that LHRH priming is the result of a direct action of LHRH on the pituitary gland. Additional studies which correlate plasma hormone levels as measured by radioimmunoassay with electron microscopic and immunocytochemical examination of pituitary gonadotropes are expected to reveal that LHRH causes both qualitative and quantitative dramatic shifts in pituitary gonadotrope cell types during LHRH priming. After the end of the LH surge, FSH remains elevated in plasma. Other experiments are designed to show that this phase of FSH release is the result of basal LHRH stimulation of pituitary gonadotropes which have been sensitized by testosterone and that this phase of FSH release is necessary for the normal growth and development of ovarian follicles.